Thermal quantitative sensory testing as a screening tool for cardiac autonomic neuropathy in patients with diabetes mellitus

Abstract Introduction Electrophysiological diagnosis of cardiac autonomic neuropathy (CAN) is based on the evaluation of cardiovascular autonomic reflex tests (CARTs). CARTs are relatively time consuming and must be performed under standardized conditions. This study aimed to determine whether thermal quantitative sensory testing (TQST) can be used as a screening tool to identify patients with diabetes at a higher risk of CAN. Methods Eighty‐five patients with diabetes and 49 healthy controls were included in the study. Neurological examination, CARTs, TQST, biochemical analyses, and neuropathy symptom questionnaires were performed. Results CAN was diagnosed in 46 patients with diabetes (54%). CAN‐positive patients with diabetes had significantly higher warm detection thresholds (WDT) and significantly lower cold detection thresholds (CDT) in all tested regions (thenar, tibia, and the dorsum of the foot). CDT on the dorsum < 21.8°C in combination with CDT on the tibia < 23.15°C showed the best diagnostic ability in CAN prediction, with 97.4 % specificity, 60.9% sensitivity, 96.6% positive predictive value, and 67.3% negative predictive value. Conclusion TQST can be used as a screening tool for CAN before CART.

the heart rate response to deep breathing, standing, and the Valsalva maneuver, and the change in blood pressure in response to standing.
CARTs are relatively time consuming and must be performed strictly under standardized conditions (Spallone et al., 2011).
Postganglionic autonomic nerves are generally formed from small nonmyelinated C-type fibers. Postganglionic parasympathetic vagal nerve fibers are typically impaired in CAN. In somatic peripheral nerves, the same C-type nerve fibers are involved in the perception of warm stimuli and in the slow component of pain. The first studies on thermal quantitative sensory testing (TQST) were published in the 1970s (Dyck et al., 1978;Fruhstorfer et al., 1976). Currently, TQST is an integral part of the diagnosis of somatic small fiber neuropathy in clinical practice within the frame of quantitative sensory testing.
Many recent studies evaluated TQST and CART in patients with diabetes mellitus (Orlov et al., 2012;Tavakoli et al., 2015;Ziegler et al., 2015). In this study, we aimed to determine whether TQST can be used as a screening tool to select patients with diabetes at a higher risk of CAN. The novelty of our study lies in the possibility of using a simple psychophysiological test (TQST) that can help clinicians select a group of patients with diabetes at higher risk of CAN instead of ad hoc use of CARTs, which must be performed strictly under standardized conditions at specialized neurophysiological departments. healthy controls completed the study, and their data were analyzed.

Study subjects
The main characteristics of the study subjects are summarized in Table 1.

Neurological and clinical examinations and laboratory parameters
All subjects underwent neurological examination that focused on neuropathic sensory and motor symptoms, and the presence of secondary diabetic complications was assessed. The blood pressure after 10 min of rest was measured twice in both the upper extremities using a calibrated tonometer. In addition to clinical status, biochemical analyses, including glucose metabolism, kidney function, liver function, thyroid function, nutritional parameters, and albuminuria, were performed.
The clinical assessment of somatic small nerve fiber function included the sharp/dull pinprick sensation (NeuroTips) and temperature sensation (TipTherm). Large nerve fiber function was assessed based on vibration thresholds using a calibrated 128-Hz tuning fork, and tactile sensation was assessed with a 10-g nylon Semmes-Weinstein monofilament.

TQST and questionnaires
TQST was performed in all subjects. The thermal stimulator

CARTs
The autonomic nervous system was assessed using the Fan study device (Schwarzer, Germany) under standardized conditions (Spallone et al., 2011

Statistical methods
Statistical analysis was performed using R package version 3.4.4 (R Core Team, 2018, Vienna, Austria). Differences in continuous variables between the studied groups were tested using two-sample tests. The

RESULTS
Patients with diabetes were divided into CAN-positive and CANnegative patients. The main clinical and laboratory parameters of the study groups are presented in Table 2 Table 3. ROC curves for CDT and WDT are shown in Figure 1.
In addition to TQST, several biochemical metabolic parameters and questionnaires focused on neuropathic symptoms were also analyzed to identify other potential markers that could contribute to the pathophysiology of CAN in diabetes mellitus.
Statistically significant differences were found between CANpositive and CAN-negative patients (p < .05) in the following variables: diastolic blood pressure (p < .001), albumin/creatinine ratio (p = .002), UENS score (p = .0228), MNSIQ score (p = .0018), and MNSIE score (p = .0025). All of the above variables had higher values in the CAN-positive patients. The diagnostic accuracy for these variables in predicting CAN was not superior compared to TQST. The highest 80.4% sensitivity but low specificity of 61.5% showed the diastolic blood pressure with the cut-off point of 76.5 mmHg. MNSIQ with the cut-off value of 5.5 points was the questionnaire with the highest specificity of 94.9% but poor 50% sensitivity. Complete data are presented in Table 4. No significant difference was found for SAS score (p = .4932) and presence of distal symmetric polyneuropathy (p = .0968).
No statistically significant difference (p > .05) between CANpositive and CAN-negative patients was found in the following biochemical parameters: fasting blood glucose, glycosylated hemoglobin, triglycerides, cholesterol, low-and high-density lipoprotein, plasma urea and creatinine, thyroid-stimulating hormone, and free thyroxine.
All study groups did not differ significantly in the postural change of systolic blood pressure during the lying-to-standing test and in presence of neuropathic pain.  (Selvarajah et al., 2015).

CONCLUSION
TQST seems to be a potential noninvasive, time-saving, and relatively simple tool to detect patients with diabetes with a higher risk of CAN.
Our study showed that CAN-positive patients had significantly higher WDT and significantly lower CDT in all tested regions (thenar, tibia, and the dorsum of the foot). The best diagnostic ability in the CAN prediction we found to be CDT on the dorsum of the foot and CDT on the tibia.
The study confirmed that TQST can be used as screening tool to identify diabetic patients for further autonomic testing in clinical practice.

CONFLICT OF INTEREST
The authors do not have any conflicts of interest to declare.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are openly available in a public repository that issues datasets with DOIs.